1. Field of the Invention
The present invention relates to a novel process for the production of pure aromatic solvents. In particular, the invention relates to a process for the production of pure aromatic solvents from dioxane contaminated aromatic streams. More particularly, the invention relates to a process for the production of Detal grade benzene from aromatic streams of the Udex process.
2. Background of the Related Art
The aromatic rich stream employed in the process of the invention is that which is left behind after the recovery of raffinate from naphtha reformate. The Udex plant feed, as reformied naphtha received from a Catalytic Reforming Unit (CRU), is drawn from the Udex charge storage tanks and is provided with gas blanketing to prevent contamination from air.
The aromatic rich feed is pumped and introduced into a counter current multistage extractor column typically at a 40.sup.th tray, after having been preheated to 120-130.degree. C. by exchanging heat with the raffinate and with hot oil in the preheater. The feed flow is controlled by a feed rate controller. The extractor column has 60 perforated trays.
The solvent, aqueous Tetra Ethylene Glycol (TTEG), which is essentially immiscible with the non-aromatic hydrocarbons of the feed is drawn from the bottom of a stripper column and is pumped to the upper section of the column at 120-130.degree. C. after being heat exchanged with stripping water. The solvent flow is controlled by a feed rate controller.
The raffinate, almost free of aromatics, is withdrawn from the top of the extractor and sent to a first stage settler after being heat exchanged with the feed and cooler water. From the first stage settler, it is sent to a second stage settler for further water washing and subsequently to a storage tank.
The aromatic rich solvent is withdrawn from the bottom of the extractor. Extractor pressure is set to maintain the hydrocarbon below its bubble point which is controlled by a split range controller at 6-7 kg/cm.sup.2. The rich solvent is introduced into a flash drum section of a column by a pressure gradient at 120-130.degree. C. through a flow controller. The flash drum operates at 2.0 to 2.5 kg/cm.sup.2 pressure. In this flash drum, hydrocarbon light ends escape together with most of the light aliphatic hydrocarbons which are subsequently condensed and received in a suitable vessel. From the stripper receiver, the condensed light ends are recycled back to the bottom section of the extractor column for further recovery of aromatics. The bottoms from the flash drum flow to the stripper section of the column where heat is supplied at the bottom by a hot oil reboiler. Bottom temperature is controlled at 140-150.degree. C. The aromatic stripping from the rich solvent is carried out and facilitated by the water which is pumped from a water receiver at the 39.sup.th tray in the form of steam and, at the bottom, in the form of water. Some aromatics leave the top of the stripper as vapor while the aromatic (extract) is drawn as a side stream from the 25.sup.th tray which, after condensation, goes to the extract receiver. The condensed aromatics, thereafter, are water washed. The washed extract flows to a clay tower feed storage tank after being pressure controlled for clay treatment and fractionation.
The stripper solvent is withdrawn as bottoms from the stripper column and returned to the extractor column. A slip stream of this solvent is preheated to around 200.degree. C. by hot oil. The regenerator bottom temperature is controlled at around 200.degree. C. The solvent vapors from the top are condensed and pumped back to the bottom of the column. A certain amount of sludge accumulates at the bottom of column which is drained out during shutdown.
The aromatic extract from the extraction section is routed to a buffer tank from which it is pumped to the clay tower after being preheated with clay tower bottoms and a hot oil preheater to 160-220.degree. C. One of the clay towers is used at a time.
The clay towers are packed with 10 MT of DCM or Korvi clay (30 to 60 mesh) having de-coloring properties. The clay treatment removes trace quantities of olefins and diolefins so as to meet the acid wash color specification for nitration-grade benzene and toluene. If the acid wash color of the out going product is no longer satisfactory, the extract will be sent through other clay towers, while the former clay tower is emptied out and loaded with a fresh charge of clay.
Clay tower pressure is controlled at 20-23 kg/cm.sup.2 to keep the extract in the liquid phase to obtain the maximum decolorizing efficiency by the clay treatment.
The decolorized product after the heat exchange with feed enters the benzene column at around 90-100.degree. C. at the 41.sup.st tray. Hot oil flow to the reboiler is adjusted to maintain bottom temperature at 142.degree. C. Tray 41.sup.st and 49.sup.th temperatures are maintained at around 95-100.degree. C. and 128-130.degree. C., respectively. These settings are essential to know the flooding condition of the column. The column top temperature is maintained at 89-92.degree. C. and the top pressure is maintained at 0.3 kg/cm.sup.2.
Column overhead vapors are condensed and received in a receiver from which settled water is drained out of a water booth and hydrocarbons are pumped as reflux which is suitably cascaded with level controllers. A drag stream is withdrawn from the pump discharge and recycled back to the stripper overhead condenser after passing through a drag stream cooler and a drag stream settler to keep the concentration of non-aromatics in the benzene to a tolerable limit.
Benzene product is withdrawn as a liquid side cut from the 7.sup.th tray which is cascaded with a differential temperature controller which maintains a differential temperature in the range of 2.5 to 3.0.degree. C. between tray 14 or 18 and tray 4. A selector switch is provided for selecting either tray 14 or 18. Benzene run down is sent to intermediate storage by a pump after cooling.
The run down benzene samples were analyzed using gas chromatography for dioxane content as per UOP-92 1. The analysis showed that all of these benzene samples were contaminated with dioxane and not suitable for producing Linear Alkyl Benzene (LAB) in the Detal unit. Pure benzene having a dioxane content less than 1 ppm is useful in the Detal process for making LAB, a surfactant intermediate.
Since dioxane has a boiling point close to that of benzene and the geometry of both of these molecules are quite similar, it is rather impossible to achieve efficient yet economic separation of dioxane at this level by distillation.
The specification of benzene suitable for the Detal process is summarized in Table-1 which indicates, along with ASTM D-4734, UOP's specification requirements which are more stringent than those of the ASTM specification.
TABLE 1 ______________________________________ Benzene Specifications for the Detal Unit PROPERTY SPECIFICATION METHOD ______________________________________ Benzene, wt % 99.90 Min ASTM D-4492 Sulfur, wt ppm 1.0 Max ASTM D-4045 or UOP-304 Thiophene, wt ppm 0.5 Max (*) ASTM D-1685 of D-4735 Toluene, wt % 0.05 Max ASTM D-4492 Nonaromatic Hydrocarbons, wt % 0.10 Max ASTM D-4492 Acid Wash Color Pass with 1 Max ASTM D-848 Acidity Not Detectable ASTM D-847 Copper Corrosion Pass (1a or 1b) ASTM D-849 Color, Pt-Co Scale 20 Max ASTM D-1209 Relative Density, 15.56/15.56.degree. C. 0.8820-0.8860 ASTM D-3505 Distillation range, .degree. C. 1.0 Max ASTM D-850 Solidification point, .degree. C. 5.45 Min ASTM D-852 Total nitrogen, wt ppm 0.3 Max (*) UOP-385 p-Dioxane, wt ppm 1.0 (*) UOP-921 ______________________________________ (*) Indicates a UOP specification in addition to or more stringent than the ASTM specification.
Detal grade benzene is conventionally produced by a sulfolane extraction process. Most present day solvent extraction units are based on solvents, such as sulfolane and pyrrolidine based solvents, which can control successfully the dioxane content of the benzene obtained from straight run naphtha to the desired limits. No method has been reported so far which can successfully control the dioxane content in benzene using TTEG as extractant.
An object of the invention is to propose a process for the production of dioxane free benzene.
Another object of the invention is to propose a process for the production of dioxane free benzene utilizing optimum operating conditions of pressure, temperature and feed flow rate for an adsorption unit under plant hydrodynamic conditions.
Yet another object of this invention is to propose a process for the production of dioxane free benzene utilizing the capacity of molecular sieves and clays for the removal of dioxane from aromatic rich streams.